The invention relates to a color display tube comprising an envelope having a display window, an electron gun for generating electron beams, a deflection system for deflecting the electron beams along electron beam paths, a color selection-electron having a large plurality of apertures, the color selection electrode having an border including an interior portion which extends towards the display window and an exterior portion extending away from the display window, and suspension means for suspending the color selection-electrode in the envelope.
U.S. Pat. No. 4,551,651 describes such a color display tube comprising an envelope including a substantially rectangular display window having an upright peripheral edge, and a substantially rectangular color selection-electrode suspended in the corners of the upright edge behind the display window, with the aid of suspension means which each include a flat spring element connected to the color selection-electrode and perpendicular to the deflected electron beam path in the relevant corner. The color selection-electrode has a border comprising a ridge extending towards the display window and a collar extending away from the display window, the centre of a cross-section through the ridge and collar being located substantially in the plane of the shadow mask.
A phenomenon occuring in a color display tube of the type defined in the opening paragraph, important for the picture quality, is what is commonly referred to as the "overall doming" of the color selection electrode. During usage, in response to impingement of the color selection electrode by electrons, the curvature of the color selection electrode changes. This results in displacement of the landing spot of the electron beam on the picture screen, a so-called landing displacement. The significance of landing displacements will be described in further detail in connection with the drawings.
In an embodiment of the prior art display tube, the collar is at such an angle to the longitudinal axis of the picture tube that electrons directly reflected by the collar fall on the color selection electrode outside the pattern of apertures in the color selection electrode, whereby the overall surface area of the color selection electrode is impinged by electron, which reduces distortion of the color selection electrode and consequently reduces overall doming effects due to thermal effects.
Experiments have however shown that in this prior art construction, overall doming does still negatively affect the picture quality.